diff --git a/sys/src/cmd/jpg/writejpg.c b/sys/src/cmd/jpg/writejpg.c
index 9a3439ce0..737f44780 100644
--- a/sys/src/cmd/jpg/writejpg.c
+++ b/sys/src/cmd/jpg/writejpg.c
@@ -878,6 +878,8 @@ writejpg0(Biobuf *fd, Image *image, Memimage *memimage,
 	case GREY2:
 	case GREY4:
 	case GREY8:
+		gflag = 1;
+		break;
 	case RGB24:
 		break;
 	default: